1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging polymers. More particularly, it concerns blends of oxygen scavenging polymers with other polymers, and especially such blends wherein the oxygen scavenging polymer is branched.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking.
A growing technique for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. The oxygen scavenger is typically a polymer capable of reacting irreversibly with oxygen that also provides adequate structural properties alone or as a blend in an oxygen scavenging layer of a packaging article. Many early approaches involved the use of polymers comprising linear alkenyl moieties, either in the polymer backbone or in groups pendant on the backbone, as oxygen scavengers. However, these approaches often suffered from off-taste and malodor concerns raised by the migration of fragments of the linear alkenyl moieties (generated by the scavenging reaction) into a packaged food or beverage.
A milestone in the development of oxygen scavenging polymers was reached by Ching et al., International Patent Publ. No. WO99/48963, which discloses the use of polymers comprising an ethylenic backbone and cycloalkenyl pendant groups, especially polymers comprising cycloalkenyl methyl acrylate or cycloalkenyl methyl methacrylate units, as oxygen scavenging polymers. The cycloalkenyl pendant groups do not generate fragments upon undergoing the scavenging reaction, and thus impart minimal off-taste and malodor to a packaged food or beverage.
It is often desirable to prepare a neat formulation of an oxygen scavenging polymer. However, the polymers comprising cycloalkenyl methyl (meth)acrylate disclosed by Ching et al. are typically formed by transesterification of acrylates or methacrylates by alcohols comprising the cycloalkenyl methyl moiety in a reactive extrusion process. Although an effective technique, producing an oxygen scavenging polymer via transesterification in a reactive extrusion is often complicated and expensive.
Production of polymers comprising cycloalkenyl methyl (meth)acrylate units by the free-radical-based direct polymerization of cycloalkenyl methyl (meth)acrylate monomers has not been shown. We found that free-radical-based bulk polymerization or solution polymerization of such monomers would lead to excessive crosslinking involving the double bonds of the cycloalkenyl groups. The result would be a polymer mass that would be difficult to process further in the formation of an oxygen scavenging packaging article from the polymer. Also, excessive consumption of the double bonds of the cycloalkenyl groups would lower the oxygen scavenging capacity of the polymer to undesirably low levels. However, it would be desirable to prepare oxygen scavenging polymers that are not excessively crosslinked, as such polymers may have desirable physical properties in certain applications.
Oxygen scavenging polymers made from ethylene-methyl acrylate copolymer via transesterification have a limited amount of oxidizable cyclohexenyl pendant groups due to the typical low conversion of the methyl acrylate monomer unit in the starting polymer to cyclohexenyl methyl acrylate, and therefore may be difficult to blend or dilute with other commercial polymers. Therefore, it is often desirable to prepare blends of oxygen scavenging polymers in order to reduce cost and improve various properties. For example, it is desirable to blend the polymers comprising cycloalkenyl methyl (meth)acrylate disclosed by Ching et al., with other polymers, such as structural polymers or oxygen barrier polymers.
In one embodiment, the present invention relates to a composition, comprising (i) a branched oxygen scavenging polymer and (ii) a second polymer. Preferably, the oxygen scavenging polymer comprises a cycloalkenyl pendant group, and the second polymer is selected from co(ethylene/methyl acrylate) (EMAC), co(ethylene/vinyl acetate) (EVA), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyamides, or co(ethylene/vinyl alcohol) (EVOH).
In another embodiment, the present invention relates to methods of preparing a neat oxygen scavenging polymer. The methods comprise (i) providing an emulsion comprising the oxygen scavenging polymer, water, and a surfactant; (ii) separating the oxygen scavenging polymer from the water, the surfactant, and other compounds that may be present, to yield a separated oxygen scavenging polymer; and (iv) drying the separated oxygen scavenging polymer, to produce the neat oxygen scavenging polymer. The separating step can involve freeze precipitation, solvent precipitation with an organic solvent, acid precipitation with an aqueous dilute strong acid solution, or spray drying. If desired, the oxygen scavenging polymer may be branched.
In a further embodiment, the present invention relates to a method of preparing a composition comprising an oxygen scavenging polymer and a second polymer. Preferably, the oxygen scavenging polymer comprises a cycloalkenyl pendant group, and the second polymer is selected from co(ethylene/methyl acrylate) (EMAC), co(ethylene/vinyl acetate) (EVA), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyamides, or co(ethylene/vinyl alcohol) (EVOH). The method comprises (a) providing (i) the oxygen scavenging polymer in neat form and (ii) the second polymer; and (b) blending the oxygen scavenging polymer in neat form and the second polymer, to form the composition. If desired, the oxygen scavenging polymer may be branched.
In one embodiment, the present invention relates to an oxygen scavenging packaging article, comprising an oxygen scavenging layer comprising a branched oxygen scavenging polymer and a second polymer. Preferably, the oxygen scavenging polymer comprises a cycloalkenyl pendant group, and the second polymer is selected from co(ethylene/methyl acrylate) (EMAC), co(ethylene/vinyl acetate) (EVA), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyamides, or co(ethylene/vinyl alcohol) (EVOH).
The present invention provides neat formulations of oxygen scavenging polymers. It also provides branched oxygen scavenging polymers that are not excessively crosslinked. In addition, it provides blends of oxygen scavenging polymers with other polymers, such as structural polymers or oxygen barrier polymers, which blends are conveniently prepared.